Measurement of the properties of a liquid or other samples may be done by sampling and laboratory analysis, using chemical or optical instrumentation, depending on what is to be measured. For example, FTIR or gas chromatography may be used. Examples of liquids measured in this way include water in hydrocarbons (fuels and oils), fuel mixtures, chemical concentration and water contamination. While laboratory analysis of samples produces accurate measurement the process is time consuming and relatively expensive. Furthermore, such laboratory analysis cannot be made in real-time or in-situ.
The use of fuel mixtures such as diesel and biodiesel results in particular problems when the composition of these mixtures is measured. Certain engines require that their fuel mixture ratio is within certain tolerances and damage or premature failure may occur if these tolerances are exceeded.
For example, the biodiesel content of diesel fuel and contamination with unrefined vegetable oil may require constant checking and verification. Unrefined vegetable oil is often used as a cheap fuel substitute for diesel trucks and cars as it is the raw material biodiesel is derived from and will combust readily in a diesel engine. However, fuel adulterated with high levels of unrefined oil may seriously damage modern diesel engines and often leads to engine failure within the warranty period of the vehicle. Also there is a requirement for quality sensors to determine the concentration of Urea in aqueous solution for use in NOx catalytic reduction systems. Also there is a requirement for quality sensors to measure oxidation of lubricating oil. Therefore, a real-time sensing mechanism is required.
A permittivity sensor driven by an oscillator may be used to measure electrical properties of a liquid in real time. These measurements may be used to infer the composition of the liquid. However, where the composition of a mixture includes different liquids having a similar electric permittivity then such a sensor may not be able to distinguish between the constituents of the liquid.
Furthermore, drift—both short term and long term—may be an inherent property of oscillators and is such that it is difficult to make absolute measurements without some form of reference system. Switching to some reference, be it a reference fluid (pure known liquid) or a dummy reference, may be used. However, it can take some time for the oscillator to stabilise when switched form one load to the reference. In the meantime, the composition of the liquid may have changed by an undesirable amount.
Electronic sensors have been developed to monitor fluids. Most of these devices are one-parameter devices that usually measure a parameter that is proportional to the dielectric constant of the sample. These are generally known as capacitive sensors. A disadvantage of these single parameter systems is that the scope of what can be detected is often rather limited and depends on the sample being very controlled, which is often unrealistic in practice.
EP2009434 describes a system for determining the concentration of biodiesel in a mixture of biodiesel and petrodiesel by measuring the amplitude and frequency of a resonant circuit. However, EP2009434 does not describe how to detect unrefined oil in the biodiesel or how to improve calibration.
Therefore, there is required a sensor that overcomes these problems and meets these requirements.